The invention relates to packet radio networks in general, and in particular to supporting mobility in packet radio networks.
Mobile communication systems have been developed because it has been necessary to be able to reach people even when they are not close to a fixed telephone terminal. As the use of various data transmission services in offices has increased, different data services have also been introduced into mobile communication systems. Portable computers enable effective data processing wherever the user moves. Mobile communication networks in turn provide an effective access network to actual data networks for the user for mobile data transmission. To realize this, data services of new kind are designed for existing and future mobile communication networks. Mobile data transmission is supported particularly well by digital mobile communication systems, such as the pan-European mobile communication system GSM (Global System for Mobile Communication).
The general packet radio service GPRS is a new service in the GSM system, and is one of the objects of the standardization work of the GSM phase 2+ at ETSI (European Telecommunication Standard Institute). The GPRS operational environment comprises one or more subnetwork service areas, which are interconnected by a GPRS backbone network. A subnetwork comprises a number of packet data service nodes SN, which in this application will be referred to as serving GPRS support nodes SGSN, each of which is connected to the GSM mobile communication network (typically to base station systems) in such a way that it can provide a packet service for mobile data terminals via several base stations, i.e. cells. The intermediate mobile communication network provides packet-switched data transmission between a support node and mobile data terminals. Different subnetworks are in turn connected to an external data network, e.g. to a public switched data network PSPDN, via GPRS gateway support nodes GGSN. The GPRS service thus allows to provide packet data transmission between mobile data terminals and external data networks when the GSM network functions as an access network. The GPRS network architecture is illustrated in FIG. 1.
In the GPRS system, layered protocol structures, known as a transmission level and a signalling level, have been defined for transmitting user information and signalling. A transmission level has a layered protocol structure providing transmission of user information together with control procedures of data transmission related to it (e.g. flow control, error detection, error correction and error recovery). A signalling level consists of protocols which are used for controlling and supporting the functions of the transmission level, such as controlling access to the GPRS network (Attach and Detach) and controlling the routing path of the established network connection in order to support the user""s mobility. FIG. 2 illustrates the signalling level of the GPRS system between an MS and an SGSN. The protocol layers of the transmission level are identical with those of FIG. 2 up to protocol layer SNDCP, above which there is a protocol of the GPRS backbone network (e.g. Internet Protocol IP) between the MS and the GGSN (instead of protocol L3MM). The protocol layers illustrated in FIG. 2 are:
Layer 3 Mobility Management (L3MM): This protocol supports the functionality of mobility management, e.g. GPRS Aftach, GPRS Detach, security, routing update, location update, activation of a PDP context, and deactivation of a PDP context.
Subnetwork Dependent Convergence Protocol (SNDCP) supports transmission of protocol data units (N-PDU) of a network layer between an MS and an SGSN. The SNDCP layer, for example, manages ciphering and compression of N-PDUs.
Logical Link Control (LLC); this layer provides a very reliable logical link. The LLC is independent of the radio interface protocols mentioned below.
LLC Relay: This function relays LLC protocol data units (PDU) between an MS-BSS interface (Um) and a BSS-SGSN interface (Gb).
Base Station Subsystem GPRS Protocol (BSSSGP): This layer transmits routing information and information related to QoS between a BSS and an SGSS.
Frame Relay, which is used over the Gb interface. A semipermanent connection for which several subscribers"" LLC PDUs are multiplexed is established between the SGSN and the BSS.
Radio Link Control (RLC): This layer provides a reliable link independent of radio solutions.
Medium Access Control (MAC): This one controls access signalling (request and grant) related to a radio channel and mapping of LLC frames onto a physical GSM channel.
With respect to the invention the most interesting protocol layers are the LCC and L3MM. The function of the LLC layer can be described as follows: the LLC layer functions above the RLC layer in the reference architecture and establishes a logical link between the MS and its serving SGSN. With respect to the function of the LCC the most important requirements are a reliable management of LCC frame relay and support for point-to point and point-to-multipoint addressing.
The service access point (SAP) of the logical link layer is a point where the LLC layer provides services for the protocols of layer 3 (SNDCP layer in FIG. 2). The link of the LLC layer is identified with a data link connection identifier (DLCI), which is transmitted in the address field of each LLC frame. The DLCI consists of two elements: Service Access Point Identifier (SAPI) and Terminal End Point Identifier (TEI). The TEI identifies a GPRS subscriber and is usually a Temporary Logical Link Identity TLLI. The TEI can also be another subscriber identity, such as an international mobile subscriber identity IMSI, but usually transmission of the IMSI on the radio path is avoided.
When a user attaches to a GPRS network, a logical link is established between the MS and the SGSN. Thus it can be said that the MS has a call in progress. This logical link has a route between the MS and the SGSN, indicated with the TLLI identifier. Thus the TLLI is a temporary identifier, the SGSN of which allocates for a certain logical link and IMSI. The SGSN sends the TLLI to the MS in connection with the establishment of a logical link, and it is used as an identifier in later signalling and data transmission over this logical link.
Data transmission over a logical link is carried out as explained in the following. The data to be transmitted to or from an MS is processed with an SNDCP function and transmitted to the LLC layer. The LLC layer inserts the data in the information field of LLC frames. The address field of a frame includes e.g. a TLLI. The LLC layer relays the data to the RLC, which deletes unnecessary information and segments the data into a form compatible with the MAC. The MAC layer activates radio resource processes in order to obtain a radio traffic path for transmission. A corresponding MAC unit on the other side of the radio traffic path receives the data and relays it upwards to the LLC layer. Finally, the data is transmitted from the LLC layer to the SNDCP, where the user data is restored completely and relayed to the next protocol layer.
The LLC layer controls transmission and retransmission of LLC frames over a logical link. Several state variables are related to the controlling at both ends of the link. In multiframe transmission such state variables include e.g. a transmission state variable V(S), acknowledgement state variable V(A), transmission sequence number N(S), receiving state variable V(R), and receiving sequence number N(R). The V(S) indicates the number of the frame to be transmitted next. The V(A) indicates the number of the last frame the opposite end has acknowledged. The V(S) shall not exceed the V(A) by more than k frames, i.e. the size of the transmission window is k. The V(R) indicates the number of the next frame that is expected to be received. The state variables are reset, i.e. set to value 0 when a logical link is being established. This is performed by using the following messages of the LAPG (Link Access Procedure on the xe2x80x9cGxe2x80x9d channel) protocol: SABM (Set Asynchronous Balanced Mode) and UA (Unnumbered Acknowledgement) or SAUM (Set Asynchronous Unbalanced Mode).
Three different MM states of the MS are typical of the mobility management (MM) of a GPRS subscriber: idle state, standby state and ready state. Each state represents a certain functionality and information level, which has been allocated to the MS and SGSN. Information sets related to these states, called MM contexts, are stored in the SGSN and MS. The context of the SGSN contains subscriber data, such as the subscriber""s IMSI, TLLI and location and routing information, etc.
In the idle state the MS cannot be reached from the GPRS network, and no dynamic information on the current state or location of the MS, i.e. on the MM context, is maintained in the network. Neither does the MS receive nor transmit data packets, in consequence of which no logical link has been established between the SGSN and the MS. If the MS is a dual-mode terminal, i.e. it can function both in the GPRS network and in the GSM network, it can be in the GSM network when functioning in the GPRS idle state. The MS can switch from the idle state to the ready state by attaching to the GPRS network, and from the standby or ready state to the idle state by detaching from the GPRS network.
In the standby and ready states the MS is attached to the GPRS network. In the GPRS network, a dynamic MM context has been created for the MS, and a logical link LLC (Logical Link Control) established between the MS and the SGSN in a protocol layer. The ready state is the actual data transmission state, in which the MS can transmit and receive user data. The MS switches from the standby state to the ready state either when the GPRS network pages the MS or when the MS initiates data transmission or signalling. The MS may remain in the ready state (for a period set with a timer) even when no user data is transmitted nor signalling performed.
In the standby and ready states the MS also has one or more PDP contexts (Packet Data Protocol), which are stored in the serving SGSN in connection with the MM context. The PDP context defines different data transmission parameters, such as the PDP type (e.g. X.25 or IP), PDP address (e.g. X.121 address), quality of service QoS and NSAPI. The MS activates the PDU context with a specific message, Activate PDP Context Request, in which it gives information on the TLLI, PDP type, PDP address, required QoS and NSAPI. When the MS roams to the area of a new SGSN, the new SGSN requests MM and PDP contexts from the old SGSN.
For mobility management, logical routing areas have been defined to the GPRS network. A routing area RA is an area defined by an operator, comprising one or more cells. Usually, one SGSN serves several routing areas. A routing area is used for determining the location of the MS in the standby state. If the location of the MS is not known in terms of a specific cell, signalling is started with a GPRS page within one routing area RA.
The MS performs a routing area update procedure in order to support mobility of a packet-switched logical link. In the READY state the MS initiates the procedure when a new cell is selected, the routing area changes or the update timer of a cyclic routing area expires. The radio network (PLMN) is arranged to transmit a sufficient amount of system information to the MS so that it can detect when it enters a new cell or a new routing area RA and to determine when it is to carry out cyclic routing area updates. The MS detects that it has entered a new cell by comparing cyclically the cell identity (Cell ID) which is stored in its MM context with the cell identity which is received from the network. Correspondingly, the MS detects that it has entered a new routing area RA by comparing the routing area identifier stored in its MM context with the routing area identifier received from the network. When the MS selects a new cell, it stores the cell identity and routing area in its MM context.
All the procedures described above (e.g. attach, detach, routing area update and activation/deactivation of the PDP context) for creating and updating MM and PDP contexts and establishing a logical link are procedures activated by the MS. In connection with a routing area update the MS, however, carries out an update to the new routing area without being able to conclude on the basis of the routing area information broadcast by cells whether the SGSN serving the new cell is the same as the SGSN that served the old cell. On the basis of the old routing area information transmitted by the MS in an update message the new SGSN detects that a routing area update is in progress between SGSNs and activates necessary interrogations to the old SGSN in order to create new MM and PDP contexts for the MS to the new SGSN. Since the SGSN has changed, the logical link should be re-established between the MS and the new SGSN. The problem is, however, that the MS does not know that the SGSN has changed. Instead, the MS may transmit data to the new SGSN, which is not able to unpack the data before MM and PDP contexts have been created on the basis of the inquiry made to the old SGSN. Furthermore, even though the contexts were already created in the new SGSN, state variables at the ends of the logical link do not match and data transmission fails, at least temporarily. If the retrieval of PDP contexts from the old SGSN fails, data transmission is prevented, since the new SGSN does not serve according to the PDP context activated earlier by the MS. The MS, however, has no information on such a situation, and it cannot start reactivation of the PDP context.
An object of the invention is to minimise and eliminate the problems and disadvantages resulting from the routing area update.
The invention relates to a method for updating a routing area in a packet radio network, the method comprising the steps of
establishing a logical link between a mobile station and a first radio support node via a radio cell,
the mobile station selecting a second radio cell on roaming, the routing area identifier broadcast by the second cell being different from the routing area identifier of the first radio cell,
sending a routing area update request to the second packet radio support node serving the second cell from the mobile station,
detecting in the second packet radio support node that the mobile station has roamed from a routing area served by a different packet radio support node,
requesting subscriber data related to the mobile station from the first packet radio support node,
sending an acknowledgement message on the routing area update from the second packet radio support node to the mobile station. The method is characterized by
initiating a signalling procedure for initializing a logical link between the mobile station and the second packet radio support node in the second packet radio support node in response to detecting that the mobile station has roamed from a routing area served by a different packet radio support node.
The basic idea of the invention is that the serving packet radio support node, which detects a routing area update performed by an unknown mobile station, initiates the establishment of a logical link by sending a link establishment message to the mobile station, the message containing the same identifier the mobile station used for itself in the routing area update request. The mobile station initializes the logical link at its own end and sends an acknowledgement to the serving packet radio support node. The initialization of the logical link performed by the mobile station may comprise resetting the state variables of the logical link and changing the new identifier possibly sent by the support node in the mobile station. Default values, for example, can be used for other possible link parameters.
The establishment message of a logical link may be any message or a sequence of messages indicating the establishment or re-initialization of a logical link. It is preferably possible to use messages similar to the ones the mobile station employs for initiating the establishment of a logical link on attaching to the network for the first time. In a preferred embodiment of the invention the serving packet radio node SGSN of the GPRS system sends a SABM command (Set Asynchronous Balanced Mode) of the LAPG protocol (Link Access Procedure on the xe2x80x9cGxe2x80x9d channel) to the mobile station, and the mobile station acknowledges it by sending an UA response (Unnumbered acknowledgement). Another preferred solution is to use the negotiation procedure of LLC link parameters of the GPRS system in such a way that the serving support node sends a XID command to the mobile station, the command including a parameter message. The mobile station sends a XID reply message including a list of the parameters it supports. When resetting of the state variables according to the invention (or another kind of initialization of the logical link) is added to this in the mobile station in response to the XID command, desirable link parameters can be provided with very little signalling.
As was stated above, in some exceptional situations the packet data protocol context (PDP) cannot be retrieved for the new support node from the old support node. This may result e.g. from the fact that the new support node does not recognize the old support node or cannot contact it, or the old support node has lost the subscriber data. In that case, in one preferred embodiment of the invention, the new support node sends information to the mobile station that it should initiate the activation procedure of a PDP context (contexts). This information can be relayed as included in the information on the change of the support node or as separate information. Without this procedure the new support node cannot function in the way defined earlier by the mobile station, and data transmission is prevented at least temporarily without the mobile station detecting this.
According to one embodiment of the invention, the new support node initiates the establishment of a logical link immediately after having detected that the support node has changed. Thus the establishment signalling of a logical link can be performed at the same time as the new support node searches for subscriber data from the old support node or the subscriber data base. An advantage resulting from this is that the mobile station does not try to transmit more data packets since it thinks that it still has a link to the old support node. Furthermore, the update of the logical link can be initiated as early as possible, which speeds up the initiation of data transmission after subscriber data have been received in the support node. If the new support node does not accept the link to the mobile station after the subscriber data have been checked, the support node disconnects the established logical link by sending a disconnect message to the mobile station. In the GPRS system a message suitable for this purpose is, for example, the DISC command (Disconnect) of the LAPG protocol.
The invention provides an effective way for re-establishing a logical link between support nodes in connection with a routing area update. The invention allows to completely avoid the update of the logical link and interruptions to data transmission in connection with an intra-support node routing area update. The last-mentioned problem would arise, for example, in such a solution in which the mobile station would always initiate re-establishment of a logical link in connection with a routing area update as a precaution.